The present invention relates generally to power amplifiers for use in power line communication systems and, more particularly, to a current limiter for use in power line communication systems that utilize phase shift keyed modulation.
Many different types of power line communication systems are known to those skilled in the art. These types of systems can be used by electric utility companies to transmit messages to remote terminals which are located at electrical consumers' residences. A coded signal is imposed on the power line by injecting it onto the 60 Hz current flowing through the power line. Remote receivers are adapted to filter the 60 Hz waveforms from the signal and demodulate the carrier signal which is generally a much higher frequency, such as 12.5 kilohertz.
It has been found that phase shift keyed (PSK) signals are well suited for power line communications systems. U.S. Pat. No. 3,911,415 which issued on Oct. 7, 1975 to Whyte discloses a distribution network power line carrier communication system and U.S. Pat. No. 3,924,223 which issued on Dec. 2, 1975 to Whyte et al. discusses a power line communication system having a protective terminating impedance arrangement. Also, U.S. Pat. Nos. 3,942,168 and 3,942,170 which issued to Whyte on Mar. 2, 1976 both disclose distribution network power line communication systems.
In applications of power line communication systems in which a transmitted message is intended to travel a considerable distance on the power line, repeaters are used for the purpose of receiving the transmitted signal and retransmitting that signal to remote receivers. U.S. Pat. No. 3,962,547 which issued on June 8, 1976 to Pattantyus discloses a repeater coupler for power line communication systems. A signal repeater for a distribution network communication system is disclosed in U.S. Pat. No. 4,210,901 which issued on July 1, 1980 to Whyte et al. When a single electric utility central communication unit is used to communicate with a large number of remote receivers, the remote receivers can be assigned addresses which permit them to distinguish messages intended for their use. U.S. Pat. No. 3,967,264 which issued on June 29, 1976 to Whyte et al. discloses a distribution network power line communication system that includes addressable interrogation and response and U.S. Pat. No. 4,250,489 which issued on Feb. 10, 1981 to Dudash et al. discloses a distribution network communication system having branch connected repeaters.
U.S. Pat. No. 4,004,110 which issued on Jan. 18, 1977 to Whyte discloses a power supply for power line carrier communication systems and U.S. Pat. No. 4,008,467 which issued on Feb. 15, 1977 to Pattantyus et al. discloses a power line carrier communication system which has an efficient carrier signal coupling apparatus.
In a phase shift keyed (PSK) carrier communication system, a base band data signal is used to modulate a higher frequency carrier signal. In a typical application of a communication system of this type, the carrier signal has a frequency of approximately 12.5 kilohertz and the base band data message is approximately 76 baud, although many other frequencies and data rates can be used. These two signals are modulated together by connecting them as inputs to an exclusive-OR gate. The result is a phase shift keyed (PSK) modulation system in which the carrier signal experiences a phase shift upon the occurrence of a leading or lagging edge of the base band signal. In this way, a receiver can demodulate the transmitted signal and decode its message. U.S. Pat. No. 4,311,964 which issued on Jan. 19, 1982 to Boykin discloses a coherent phase shift keyed (PSK) demodulator for a power line communication system. U.S. Pat. No. 4,379,284 which issued on Apr. 5, 1983 to Boykin discloses an improved coherent phase shift keyed (PSK) demodulator for power line communication systems. A coherent phase shift keyed demodulator is disclosed in U.S. Pat. No. 4,514,697 which issued on Apr. 30, 1985 to York. An improved coherent phase shift keyed demodulator is disclosed in U.S. Pat. No. 4,516,079 which issued on May 7, 1985 to York.
A receiver for a distribution network power line communication system is disclosed in U.S. Pat. No. 4,355,303 which issued on Oct. 19, 1982 to Phillips et al. and U.S. Pat. No. 4,357,598 which issued on Nov. 2, 1982 to Melvin discloses a three-phase power distribution network communication system. Some repeaters and load management terminals can utilize transceivers which are capable of both transmitting and receiving power line communication signals. Such a transceiver is disclosed in U.S. patent application Ser. No. 575,125 which was filed on Jan. 30, 1984 by Shuey and assigned to the assignee of the present application.
After a coded message modulates the carrier signal, the resulting PSK square wave signal is amplified and injected onto the power line as a sinusoidal current waveform. This square wave signal can be amplified by use of a switching type transmitter. Known amplifiers utilize metal-oxide semiconductor field-effect transistors (MOSFET) for switching. The MOSFETs are devices that consist of diffused source and drain regions on either side of a p or n channel region, and a gate electrode insulated from the channel by silicon oxide. When a control voltage is applied to the gate, the channel is converted to the same type of semiconductor as the source and drain. This eliminates part of the pn junction and permits current to be established between the source and the drain. Functionally, the main difference between a MOSFET and a bipolar transistor is that the source and drain of the MOSFET are interchangeable, unlike the emitter and collector of the bipolar transistor. This type of amplifier circuit responds to changes in a voltage signal and its output is an amplified current signal which is synchronized with the frequency and phase of its input.
In a power amplifier used in conjunction with a power line communication system, the input voltage signal is a modulated carrier signal and the output current signal is injected onto the power line for transmission to remote receivers. The magnitude of the amplifier's current output signal is a function of the condition and characteristics of the power line itself. Under certain disadvantageous power line conditions, the output current signal can fluctuate widely in magnitude with potentially destructive results. Therefore, some current limiting means must be used to protect the components of the amplifier.
One possible solution to this potential problem is to provide the amplifier with its own known output burden. However, this current limiting technique is inefficient because it utilizes components that dissipate significant power and these components also increase the cost of the amplifier itself. Therefore, a simplified current limiting technique is beneficial for use with power line communication system amplifiers.
The present invention provides a means for altering the voltage input to the power amplifier in such a way so as to reduce its output current signal. The current limiter of the present invention responds to the current magnitude of the power amplifier's output current signal and, when this output current signal exceeds a preselected threshold level, a modulated carrier signal, which is the input to the power amplifier, is altered in such a way so as to reduce the tendency of the power amplifier to output current signals that exceed the threshold on subsequent transmissions.
The present invention is coupled to an output line of the power amplifier by a saturable core current transformer. The secondary current of this current transformer is passed through a full wave rectifier and the rectified signal is compared with a reference signal provided by a resistive voltage divider. These two signals are compared by a comparator and, if the rectified current signal exceeds the reference signal, a logically high signal is sent to an exclusive-OR gate which has, as its other input, the modulated carrier wave signal intended for amplification and transmission. The exclusive-OR gate combines the modulated carrier signal, which contains the coded message to be output, with the signal from the present invention which represents a logically high signal when the current threshold has been exceeded. Therefore, the present invention has the result of removing segments from the logically high portions of the modulated carrier signal and also inserting segments onto the logically low portions of the modulated carrier signal when the output current signal of the power amplifier exceeds the preselected current threshold.
Therefore, the modulated carrier signal which is input to the power amplifier is altered in such a way so as to distort the resulting current signal output and reduce its maximum current levels. Although the present invention has the effect of slightly distorting the shape of the current output signal of the power amplifier during current limiting, its fundamental phase characteristics remain unchanged. Therefore, the present invention provides a current limiter that protects the components of the power amplifier, but does not alter the coded message that is injected onto the power line system.